This invention relates to carboxyl modified olefinic copolymer compositions which are improved additives for unsaturated polyester compositions. More specifically, the invention relates to ethylene/alphaolefin copolymers which improve the shrinkage and surface smoothness characteristics of an unsaturated resin mixture.
Molding compositions formulated of curable or thermosetting unsaturated polyester resins have enjoyed widespread use, specifically for making fiber reinforced thermoset plastics. Fiber reinforced molding compositions are specifically employed to provide prepreg mats, sheet molding compounds (SMC), and bulk molding compounds (BMC) which are materials commonly used for preparing reinforced cured shaped articles by either injection molding or press molding. The molding compositions are typically mixtures of the unsaturated polyester resins, fillers, fiber reinforcers, initiators, thermoplastic polymers and other minor additives such as mold release agents, thickeners, and pigments.
A continuous problem area in the thermosetting polyester resin art has been encountered in the curing step of the unsaturated polyester resin composition. When cured in a condensation solution or dispersion of an unsaturated monomer solvent, such as styrene, the molded product has a tendency to shrink and crack, as well as a propensity to have a dull surface. The shrinkage problem is particularly acute when the cured resin has fiber reinforcement. In this case the shrinkage of the curing resin produces an imprint of the fiber on the surface of the molded article with resulting detrimental effects to the surface smoothness of the formed article. In this regard, the use of low profile additives or thermoplastic polymers has been a significant contribution to the commercialization of polyester molding compositions by improving the surface effects and dimensional stability of the cured products.
Curing of the molding compositions (e.g., SMC and BMC) generally takes place at elevated temperatures. Generally, the shrinkage reduction effect of the low profile additive is attributed to the fact that the low profile additive (LPA) or thermoplastic polymer becomes less soluble in the resin at elevated temperatures, causing a partial phase separation. A two-phase mixture results with the curing polyester resin constituting the continuous phase and the thermoplastic or low profile additive constituting the distributed phase. The discontinuous phase thermoplastic polymer within the composite has been found to decrease the amount of shrinkage that occurs upon curing of the composite. It is generally observed that the greater the thermoplastic polymer content the greater the shrinkage reduction effect of the thermoplastic resin or low profile additive.
Many attempts have been made in the art to improve the surface characteristics of unsaturated polyester resins. One is disclosed in U.S. Pat. No. 4,100,224. This patent proposes the use of thermoplastic polymers, and specifically discusses a thermoplastic graft elastomer which is the reaction product of a styrene copolymer and monomers such as styrene or acrylonitrile. Although this composition decreases shrinkage, the reduction is marginal.
The use of ethylene/propylene copolymers and terpolymers in various forms, in unsaturated polyester molding compositions, has been recognized, for example, in U.S. Pat. No. 4,100,224 discussed above and in U.S. Pat. Nos. 3,880,950; 4,258,143; 4,299,927. U.S. Pat. No. 3,880,950 discusses the use of a microgel polymer in conjuntion with a thermoplastic polymer in a polyester resin composition. Either or both of the microgel polymer or the thermoplastic polymer may contain carboxyl groups. The preferred microgel polymer includes a copolymerization product of alpha, beta unsaturated carboxylic acids with at least one of styrene, methyl methacrylate and acrylonitrile in the presence of polybutadiene or polyisoprene. The microgel polymer is described as having 0.1-10% by weight carboxyl groups.
U.S. Pat. Nos. 4,258,143 and 4,299,927 disclose thermoplastic low profile additives of a carboxyl modified polyolefin. Specifically exemplified are the homopolymers polyethylene, polypropylene and polybutylene. Generally, a wide range of thermoplastic homo- and copolymers are discussed including ethylene/alphaolefin copolymers. The copolymer and homopolymer compositions are described as having a melt viscosity of 100-40,000 at 175.degree. C. The lowest percentage shrinkage obtained by the use of the polyolefin modifier disclosed was 2.88% as compared to a 3.5% shrinkage in a control polyester without a thermoplastic polymer modifier.
Although various thermoplastic resins have been proposed in the art, the search still continues for improved low profile additives. Therefore, even though low profile additives are generally known, and specifically, they are known for their reduction of shrinkage in curing compositions, there still remains considerable room for improvement in terms of both shrinkage control and improvement in surface characteristics of the final molded article formed from molding compositions containing low profile additives.